Recent years have witnessed the growing importance of nonaqueous electrolyte secondary batteries, for instance lithium secondary batteries and nickel hydride batteries, as vehicle-mounted power sources, and as power sources that are provided in, for instance, personal computers and portable terminals. In particular, lithium secondary batteries, which are lightweight and afford high energy density, are expected to be used as preferred high-output power sources installed in vehicles. In lithium secondary batteries, charge and discharge take place through exchange of Li ions between a positive electrode and a negative electrode.
In a typical configuration of such lithium ion batteries, the battery is provided with an electrode body (wound electrode body) having a structure wherein sheet-shaped electrodes are wound spirally. Such a wound electrode body is formed by winding, to a spiral shape, a negative electrode sheet in which a negative electrode active material layer containing a negative electrode active material is held on both faces of a negative electrode collector, and a positive electrode sheet in which a positive electrode active material layer containing a positive electrode active material is held on both faces of a positive electrode collector, with a separator sheet interposed in between.
Examples of the negative electrode collector used in the negative electrode include, for instance, a sheet-shaped or foil-like member having copper or a copper alloy as a main constituent. Examples of the negative electrode active material that is used in the negative electrode include, for instance, a graphite material such as natural graphite. As illustrated in FIG. 9, such graphite has a layered structure, and is formed through stacking of multiple layers in each of which carbon atoms are planarly spread forming a network structure. During charging, Li ions intrude through edge faces (faces resulting from layer superposition) 3 of layers 2, and diffuse between the layers. During discharge, Li ions can be released through the edge faces 3 of the layers 2. The electric resistivity of graphite 1 is lower in the surface direction of the layers than in the stacking direction of the layers. As a result, there forms a conduction path 4 of detoured electrons along the surface direction of the layers.
Technologies have been proposed that involve magnetically orienting the graphite that is used in such lithium secondary batteries. For instance, Patent Literature 1 discloses the feature of imparting orientation in such a manner that the (002) plane of graphite becomes substantially perpendicular to a collector 5, in a magnetic field, followed by solidification, during formation of a negative electrode. In this case, as illustrated in FIG. 10, the edge faces 3 of the layers 2 become oriented towards the positive electrode; as a result, Li ions become intercalated and deintercalated smoothly while the conduction path 4 of electrons becomes shorter. Therefore, this allows enhancing the electron conductivity of a negative electrode sheet 6.